A flexible coupling is necessary since it is practically impossible to achieve and maintain perfect alignment of coupled rotating shafts. During initial assembly and installation, precise alignment of the shaft axes is not only difficult to achieve but in many cases it is economically unfeasible. During operation, alignment is even more difficult to maintain. Shaft misalignment - caused by uneven bearing wear, flexure of structural members, settling of foundations, thermal expansion, shaft deflection and other factors - is an operating certainty. Because these factors are extremely difficult to control, a flexible coupling serves as an ideal answer to compensate or minimize the effects of unavoidable misalignment and end movement of coupled shafts.

A flexible couplingmust provide three basic functions:

1. Physically couple together two rotating shafts for efficient transmission of mechanical power, transferring the torque of one shaft to the other, directly and with constant velocity.

2. Compensate for all types of misalignment between rotating, connected shafts without inducing abnormal stresses and loads on connected equipment, and without tangible loss of power.

3. Compensate for end or axial movement of the coupled shafts, preventing either shaft from exerting excessive thrust on the other and allowing each to rotate in its normal position.